Maybe QtWebEngine component was not installed correctly.
Go to your Qt 5.8 binaries folder (in my case is C:\Qt\Qt5.8.0\5.8\msvc2015\bin) and check if you have the QtWebEngine components there, the image below shows those components highlighted in red.

If you don't have those components installed in your file system open the Maintenance Tool (located in C:\Qt\Qt5.8.0 in my case) and add it those components checking the option highlighted in red in the image below